This invention relates to brush wear monitors for dynamoelectric machines and more particularly to a brush wear monitor having an improved arrangement for indicating the wear status of brushes.
Brushes are employed in DC motors and generators to transfer electric current between stationary machine components and a rotating commmutator. The commutator is constructed of copper, a relatively soft metal. The brushes must be constructed of material which will itself wear as a result of contact with the rotating commutator, rather than causing wear of the commutator. Typically, brushes are made of combinations of carbon, graphite and perhaps a small amount of metal. Although brush wear is necessary, brushes must not be allowed to wear down completely or damage to the motor or generator may result. To avoid this problem, industrial users of dynamoelectric machines first employed inspection schedules to discover and replace excessively worn brushes. Brush wear monitoring systems have also been employed and usually permit less frequent inspections without increasing the risk of complete brush failure.
A brush wear monitor system of particular advantage is disclosed in U.S. Pat. No. 4,333,095 assigned to the Assignee of the present invention. The brush wear monitor system described in U.S. Pat. No. 4,333,095 includes an indicator for receiving a "short brush" signal and providing a visual warning of the condition. The commercial embodiment of the monitor system included a warning light which was illuminated when the "short brush" signal was received, and further included a manual push button test feature which allowed a user to test the operation of the warning light. This system was subject to certain disadvantages, however, in that if the warning light or monitor system failed, or if the power input to the monitor system was disrupted, the user would receive no warning of excessive brush wear and would have no notice of failure without periodically performing a manual check.
Another brush wear monitor system on the market includes a pair of lights, one to indicate "normal" operation and the other to indicate excessive brush wear. There is no checking feature, however, to detect a failure of the warning light so that a "short brush" signal could be overlooked.